{-# LANGUAGE MagicHash, UnboxedTuples, Rank2Types, FlexibleInstances,
MultiParamTypeClasses, UndecidableInstances, DoRec #-}{- |
Module : Control.Monad.ST.Trans
Copyright : Josef Svenningsson 2008-2010
(c) The University of Glasgow, 1994-2000
License : BSD
Maintainer : josef.svenningsson@gmail.com
Stability : experimental
Portability : non-portable (GHC Extensions)
This module provides the implementation of the 'STT' type for those
occasions where it's needed in order to implement new liftings through
operations in other monads.
Warning! This monad transformer should not be used with monads that
can contain multiple answers, like the list monad. The reason is that
the will be duplicated across the different answers and this cause
Bad Things to happen (such as loss of referential transparency). Safe
monads include the monads State, Reader, Writer, Maybe and
combinations of their corresponding monad transformers.
-}moduleControl.Monad.ST.Trans.InternalwhereimportGHC.Base-- | 'STT' is the monad transformer providing polymorphic updateable referencesnewtypeSTTsma=STT(State#s->m(STTRetsa))unSTT::STTsma->(State#s->m(STTRetsa))unSTT(STTf)=f-- | 'STTRet' is needed to encapsulate the unboxed state token that GHC passes-- around. This type is essentially a pair, but an ordinary pair is not-- not allowed to contain unboxed types.dataSTTRetsa=STTRet(State#s)a